1. Field of the Invention
The present invention relates to an oxide superconductor wire and a superconducting coil.
2. Description of the Related Art
A metal superconductor wire such as NbTi which is used in the conventional art is provided, for example, in round or angular form, and the degree of freedom for the shape thereof is high.
On the other hand, in an oxide superconductor of Bi (bismuth), Y (yttrium), or the like in which a critical temperature is approximately 90 K to 100 K, an oxide superconductor layer is formed of a ceramic.
In addition, it is preferable that the structure of a wire formed of such an oxide superconductor have a tape shape having a high aspect ratio.
An oxide superconductor layer is formed of one kind of ceramics.
A superconductor wire of a rare earth oxide which is known as an yttrium-based oxide has a structure in which multiple thin films are laminated on a metal substrate provided as a tension member.
For example, an oxide superconductor wire of a rare earth oxide has a structure in which an oxide superconductor layer is laminated on a tape-shaped metal substrate through an intermediate layer having a controlled crystal orientation, and a stabilizing layer formed of a high-conductivity material such as Cu is laminated on the oxide superconductor layer.
Using the above-described rare earth oxide-based superconductor wire, a superconducting coil is manufactured.
In addition, after coiling the tape-shaped oxide superconductor wire, the coil is hardened with an impregnating resin to improve the strength of the superconducting coil.
However, the metal substrate which is applied to the oxide superconductor wire is formed of a nickel alloy which is useful as a tensile member, for example, HASTELLOY (trade name, manufactured by Haynes International Inc.), and the stabilizing layer is formed of a high-conductivity material such as Cu.
In contrast, the impregnating resin surrounding the coil is formed of a resin.
Therefore, stress is generated due to a difference in linear expansion coefficient or a difference in shrinkage ratio between these materials.
For example, since the superconducting coil is cooled to a liquid nitrogen temperature or lower and used at a low temperature, the linear expansion coefficients of these materials are non-linear.
Therefore, when a comparison is performed using a shrinkage ratio in which a ratio of the length at room temperature to the length at a low temperature is represented by a percentage, a peel stress is applied in a thickness direction of the superconductor wire during cooling.
Therefore, when a superconducting coil is prepared and has a resin-impregnated structure, a peel stress is applied in a thickness direction of a superconductor wire during cooling.
As a result, after forming the superconducting coil, superconductivity may deteriorate.
In order to avoid this peel stress problem caused by a difference in shrinkage ratio,
Japanese Unexamined Patent Application, First Publication No. 2011-198469 discloses a technique of coating the entire circumferential surface of a superconductor wire with an insulating coating layer and forming a release material layer on only part of a surface of the insulating coating layer.
FIG. 9 illustrates a superconductor composite wire 103 disclosed in Japanese Unexamined Patent Application, First Publication No. 2011-198469 in which the entire circumference of an oxide superconductor wire 100 formed of a rectangular material is coated with an insulating material layer 101, and a release material layer 102 is provided along one surface of the insulating material layer 101.
In addition, Japanese Unexamined Patent Application, First Publication No. 2011-198469 discloses a superconducting coil 105 which is formed by coiling the superconductor composite wire 103 illustrated in FIG. 9 and then hardening the obtained coil with a cured resin layer 104 illustrated in FIG. 10 which is impregnated and cured with a thermosetting synthetic resin such as an epoxy resin.
Japanese Unexamined Patent Application, First Publication No. 2011-198469 discloses the fact that the insulating material layer 101 is formed of polyesters or polyurethanes and the fact that the release material layer 102 is formed of waxes, silicone oils, or various resins.
In addition, Japanese Unexamined Patent Application, First Publication No. 2010-267835 discloses a coiling technique of lap-winding, when a superconductor wire is wound and coiled, an insulating tape around a tape-shaped superconductor wire.
It can be presumed that the techniques disclosed in PATENT Japanese Unexamined Patent Application, First Publication No. 2011-198469 and Japanese Unexamined Patent Application, First Publication No. 2010-267835 are effective for cases where the superconductivity of a superconducting coil may deteriorate due to a peel stress which is applied in the thickness direction of a superconductor wire during the cooling of the superconducting coil.
However, when a superconductor wire is formed in a coil shape, it is necessary that the release material layer 102 be newly formed. Therefore, the number of processes during coiling is increased.
In addition, in the technique disclosed in Japanese Unexamined Patent Application, First Publication No. 2011-198469, the thickness of the release material layer 102 is not disclosed. However, when the release material layer 102 is thick, an occupancy ratio of the release material layer 102 in a cross-sectional area of the superconducting coil is increased, and an exclusive area of the superconductor wire is decreased correspondingly.
Therefore, it is considered that the thickness of the release material layer 102 is preferably less than or equal to approximately 10 μm.
However, from the viewpoint of coiling workability, it is difficult to wind the release material layer 102 having a thickness of approximately 10 μm or less along with the superconductor wire during coiling.
In addition, when a method of uniformly forming the release material layer 102 having a thickness of 10 μm or less is not found, thick portions and thin portions are mixed in the release material layer 102.
As a result, strain caused by peel stress may be locally concentrated on a part of the superconducting coil.
Accordingly, it is considered that the superconductor wire may deteriorate.
The present invention has been made in consideration of such circumstances of the conventional art, and an object thereof is to provide an oxide superconductor wire having a structure in which the application of stress to the superconductor wire is suppressed when a superconducting coil is used in a cooled state and having a structure in which deterioration in superconductivity is suppressed; and a superconducting coil which is formed using the oxide superconductor wire.